The present invention relates to an apparatus for holding a substrate to be polished for use in chemical mechanical polishing (CMP) to flatten the surface of a semiconductor substrate or a liquid-crystal substrate. More particularly, it relates to an apparatus for holding a substrate to be polished whereby the substrate is held and the held substrate is pressed against a polishing pad.
From the 1990s, as semiconductor and liquid-crystal substrates processed by CMP technology have had larger diameters on the order of 10 cm or more, there have been increasing tendencies toward single-wafer polishing. In the case of polishing a semiconductor substrate, in particular, equal polishing should be performed with respect to the entire surface thereof since a design rule of 0.5 .mu.m or less has been used to define extremely small lines and spaces on the semiconductor substrate.
Below, a description will be given to an apparatus for polishing a substrate in which a conventional apparatus for holding the substrate to be polished is employed with reference to the drawings.
FIG. 6 schematically shows the structure of the conventional apparatus for polishing a substrate, in which is shown a table 101 including: a pad table 101a having a flat surface which is made of a rigid member; a rotary shaft 101b extending downwardly from the back face of the pad table 101a; and rotating means (not shown) for rotating the rotary shaft 101b. To the top surface of the pad table 101a of the table 101 is adhered an elastic polishing pad 102. Above the polishing pad 102 is provided a substrate holding head 104 which holds and rotates a substrate 103. The substrate 103 is pressed against the polishing pad 102 while being rotated by the substrate holding head 104. Abrasive slurry 105 in a prescribed amount is supplied dropwise from an abrasive supply pipe 106 onto the polishing pad 102.
In the polishing apparatus thus constituted, the table 101 is rotated to rotate the polishing pad 102, while the abrasive slurry 105 is supplied onto the polishing pad 102, during which the substrate 103 held by the substrate holding head 104 is pressed against the rotating polishing pad 102 so that a surface of the substrate 103 is polished under pressure and at a relative speed.
In this process, if the surface of the substrate 103 being polished is rugged, the polishing of the protruding portions thereof is enhanced since their contact pressure with the polishing pad 102 is high. On the other hand, the polishing of the recessed portions thereof is inhibited since their contact pressure with the polishing pad 102 is low. Consequently, the surface of the substrate 103 to be polished becomes less rugged and more smooth. This polishing technology is introduced in, for example, "Monthly, Semiconductor World" p.p.,58 to 59, in "Solid State Technology" July, 1992/ the Japanese version, p.p.,2 to 37, and the like.
In the above conventional polishing apparatus, there has been a problem as follows: the substrate 103 held by the substrate holding head 104 is pressed against the polishing pad 102 with an equal force. However, if the pad table 101a of the table 101 has a warped surface and the polishing pad 102 is elastically deformed, or if the thickness of the substrate 103 has variations, the contacting force between the substrate 103 and the polishing pad 102 is not equal but differs from one portion to another within the surface of the substrate 103, resulting in an unequal amount of polishing with respect to the substrate 103.
Then, as shown in FIG. 7, there has been considered an apparatus for holding a substrate to be polished which comprises: a substrate holding head 111 having a fluid supply path 111a which allows a fluid under pressure having flown from one end thereof to flow out of an outlet port at the other end thereof; an annular sealing member 112 made of an elastic material fasten to the peripheral portion of the back face of the substrate holding head 111; and a guiding member 113 fixed outside the annular sealing member 112 at the underside of the substrate holding head 111. According to the apparatus for holding a substrate to be polished, a fluid under pressure introduced form one end of the fluid supply path 111a is supplied from the outlet port to a space 114. Thus, the pressing force of the fluid under pressure supplied to the space 114 presses the substrate 103 to be polished from its upper face against the polishing pad 102. This results in the pressing of the substrate 103 to be polished against the pressing pad 103 with an equal pressing force within the surface of the substrate 103.
However, in the above apparatus for holding a substrate to be polished, the fluid under pressure supplied to the space 114 flows to the outside from the space 114 defined by the substrate holding head 111, annular sealing member 112, and the substrate 103 through the clearance between the annular sealing member 112 and the substrate 103. When an air under pressure flows out, there has arisen a new problem as follows:
First, the equal pressing with respect to the substrate 103 to be polished cannot be performed, resulting in unequal polishing with respect to the surface of the substrate 103. That is, the air under pressure supplied to the space 114 does not flow out from the clearance between the annular sealing member 112 and the substrate 103 evenly in the circumferential direction, but flows out from the discharge part 115 which is a part of the clearance between the annular sealing member 112 and the substrate 103 as shown in FIG. 8. In FIG. 8, the substrate 103 to be polished is shown larger in thickness than actual thickness for convenience in showing the drawing. Therefore, in the region, in the space 114, in proximity to the discharge part 115, which is a part of the above clearance, from which the air under pressure flows out, the pressure of the air approaches atmospheric pressure (the pressure of the air reduces), and hence the pressure exerted on the substrate 103 to press it reduces as compared with the region from which the air under pressure does not flow out. Accordingly, it becomes impossible to press the substrate 103 to be polished with an equal force.
Secondly, in the course of polishing, there has arisen a problem that the substrate 103 to be polished thrusts out of the substrate holding head 111. That is, the substrate holding head 111 rotates and the air under pressure flows out of the discharge part 115 which is a part of the clearance between the annular sealing member 112 and the substrate 103 to be polished, and hence the substrate holding head 111 vertically vibrates. The substrate 103 to be polished is also subjected to a frictional force from the polishing pad 102, and hence the substrate 103 to be polished is not allowed to rotate at the same rate as that of the substrate holding head 111. These two phenomena causes the substrate 103 to be polished to thrust out of the substrate holding head 111.